Controlled venting of abandoned oil/natural gas wells is sometimes required by regulatory agencies; or may be desired by well owners or property developers. The purpose of controlled venting of an abandoned oil or gas well is to allow gases and hydrocarbon vapors that may come up through or around the abandoned well casing(s) to vent to the atmosphere in a controlled manner.
Casings, as described above, may be either: (1) production casings which are strings of pipe through which gas and/or oil was delivered to the surface from the depth at which it originates in the earth; or, (2) conductor casings which are larger diameter, shorter length casing strings generally used around the production casing to prevent earth caving prior to and during installation of the production casing string.
One agency which has set forth requirements for vent hoods is the State of California (USA), Department of Conservation, Division of Oil and Gas and Geothermal Resources (DOGGR). Their requirements specify minimum dimensions for a vent hood, but do not identify materials of construction. The required minimum dimensions are also intended to provide a volume estimate for the interior or cavity.
Various materials have been used for vent hoods in the prior art. Some of the materials and their method of installation include the following:
A. Brick Cone.
The brick work would begin with a ring of bricks laid to the appropriate diameter on an earth shoulder around an abandoned well. Construction of the brick work would progress with additional courses of brick laid in ever smaller diameters until a cone would be formed around the well. During its construction, the interior of the cone would be filled with rock. A vertical pipe riser would be installed coming out of the top of the cone and run to an above-surface vent riser. Disadvantages of this method include that it is labor intensive; the quality of the field work is variable; and bricks are not considered a perfect methane gas barrier because of the high potential for the gas to escape through joints or cracked masonry units.
B. Fiberglass Cone.
Normal construction would include placing a mound of rock over the abandoned well and then the fiberglass cone on top of the rock mound. Fiberglass is a good gas/vapor barrier. However, there are disadvantages associated with using fiberglass. The shape of the cone may not match the shape of the mound; and cracking of the cone can occur from soil overburden, particularly where the cone is not completely supported by the rock mound. Cracking may also occur at the top of the cone from distress where the vent pipe extends out of the cone, because of the nature of the fiberglass material. Additionally, when a cone is not filled completely with rock, the surrounding soil can eventually migrate underneath the hood into spaces not filled with rock. This migration will then leave soil voids outside the vent cone. These soil voids can eventually lead to undesired settlement observed at the surface. Settlement can cause structural damage, particularly when the abandoned well is in close proximity to buildings or if utility or other conduits are located within the settlement zone.
C. HDPE (High Density PolyEthylene) tank.
A full tank made of heavy HDPE would be cut in half. A half tank would be placed over the well and filled with rock. HDPE is an acceptable gas/vapor barrier. Heavy walled HDPE is somewhat flexible and less likely to crack than fiberglass. Distress can still occur where the vent pipe attaches to the HDPE body. When a cone is not completely full of rock the surrounding soil can eventually migrate underneath the hood into spaces not filled with rock with the same results as described above under fiberglass cones.
D. HDPE plastic sheeting.
HDPE plastic sheeting is very flexible and would not be susceptible to crack failure. The vertical vent pipe would be "booted" to the top portion of the sheeting and would not be susceptible to cracking failure. Disadvantages are that the field HDPE membrane installation involves special equipment for thermal welding, is labor intensive and must be carefully inspected to insure against leaks in seams. The material must also be carefully protected against puncture from the rock fill, and also from the earth backfill. In addition the flexible HDPE membrane does not hold its shape. The normal construction practice has been to carefully line an excavation with the membrane, and then backfill with rock; and finally weld on a lid section of membrane. Here, proper function is dependant upon the accuracy of the excavation. Flexible membrane liners have no structural strength and could tear or puncture. The excavation may also be larger than necessary. If this occurs, more membrane and rock material is required and the cost of installation increases. Also the length of time required to install a flexible membrane can be a disadvantage for a project which requires a fast schedule.
One disadvantage common to all of the prior art described is that all require special on-site inspection during the installation process to observe for cracks and other defects which would affect vent hood integrity.